The present invention relates generally to vertebral fixation or defect devices, and more particularly to cervical fixation devices for insertion into an intervertebral space.
As shown in prior art FIGS. 7A-11, it is known in the prior art that the spine 100, also known as the spinal column or vertebral column, supports the upper body, allows head, neck, and trunk motion, and includes twenty-four moveable vertebrae 101 including seven cervical vertebrae 102, twelve thoracic vertebrae 104, and five lumbar vertebrae 106, which extend from the skull to the sacrum.
Referring to FIG. 8, with the exception of the first, upper most cervical vertebra 102, each vertebra 101 has a vertebral body 108, a lamina 110, a spinous process 112, as well as facet structures 114 (which form facet joints), two transverse processes 116, and two pedicles 118, one on each side. Each individual vertebra 101 has a large foramen 120, which forms the spinal canal (not shown) when the vertebrae 101 are in their normal anatomical position forming the spine 100. The spinal cord and major nerve fiber groups pass through and are protected by the spinal canal. A strong fibrous membrane, the dura mater (not shown), also known as the dura, surrounds the spinal cord, nerve fibers, and fluid in the spinal canal.
Referring now to FIGS. 8 and 9, each pair of adjacent vertebrae 101 along with interconnecting soft tissues and an intervertebral disk 128 constitutes a motion segment 122, also known as a functional spinal unit, and the combined motions of many of such motion segments constitute overall spinal motion at any one time. The joining of two vertebrae 101 also creates two neuroforaminae 124, also known as intervertebral foraminae, one on each side, each of which is bordered by a facet joint 126 dorsally, a pedicle 118 superiorly, a pedicle 118 inferiorly, and an intervertebral disk 128 ventrally. Each neuroforamina 124 allows passage of large nerve roots (not shown) and associated blood vessels (not shown). The intervertebral disk 128 resides in the space between adjacent vertebral bodies, the intervertebral space 130, also known as the interbody space or disk space. The level of each particular intervertebral space 130 and intervertebral disk 128 is identified by naming the vertebrae 101 superior and inferior to it, for example L 4-5 in the case of the intervertebral space 130 and intervertebral disk 128 between the fourth and fifth lumbar vertebrae 106.
Referring to FIG. 10, each intervertebral disk 128 includes a collection of peripheral concentric rings comprised of strong ligaments known as annular ligaments 132, also known as the annulus, and a softer central area of normally well hydrated material known as the nucleus 134. The annular ligaments 132 are arranged at different angles in alternate layers such that they provide support and stability, resisting excessive vertebral body 108 rotation and axial motion when proper tension is maintained. Although described by some as a cushion, the nucleus 134 is relatively incompressible in a young healthy spine, and thus its major role is to provide support and tension of the annular ligaments 132 to maintain stability while allowing a limited range of motion.
Referring now to FIG. 11, with the exception of the first cervical vertebra (not shown), in a cervical vertebra 102 the inferior surface of the vertebral body 108 is concave (as shown in phantom), while the superior surfaces are flatter centrally and curl laterally to form the uncinate processes, which partially articulate with the inferior surface of the vertebral body of the adjacent superior vertebra. Owing to the shapes of the inferior and superior surfaces of the vertebral bodies 108, the cervical intervertebral space 130 and intervertebral disk 128 are convex on the superior side but flatter on the inferior side. Cervical intervertebral disks are generally flatter and thinner than lumbar and thoracic intervertebral disks.
Situations arise in which one or more cervical vertebrae 102 do not have adequate support or stability, which can lead to pain, deformity, stenosis of spinal canal or neuroforamina, and impairment or loss of nerve function. In some cases, surgical spine fusion is considered. Spine fusion is a process of growing bone between two or more adjacent vertebrae 101 such that the adjacent vertebrae 101 will move only in unison. This process involves placing bone, or material to guide or stimulate bone growth, in proximity to exposed bone of the vertebrae 101, and then allowing time for new bone to grow and form a structurally strong connection, or fusion, between the vertebrae 101. The earliest such procedures took place approximately a century ago, and the procedures have developed over many years, including various attempts to fuse posterior structures of the spine such as the spinous process 112, lamina 110, facet joint 114, and transverse processes 116.
Recently, there has been more interest in fusion involving bone growth directly between adjacent vertebral bodies 108. Large amounts of well vascularized bone are in close proximity, there is a large surface area available, and the inherent compression force applied between vertebral bodies by muscle tension and the upright position of the human body enhances bone formation and strength. The intervertebral disk space 130 has therefore become a major focus in interbody fusion surgery. The intervertebral disk space 130 is cleaned as much as possible, and cartilage and abnormal surface bone, also known as endplate bone, from adjacent vertebral bodies 108 is removed, after which material is placed in the space to promote fusion. However, loose bone fragments do not provide structural support and therefore fusion is often unsuccessful. Structural bone grafts from the patient or donors have been successful, but may give rise to pain and complications if from the patient, and risk of disease transmission if from a donor.
Vertebral defect devices are increasingly used to assist with fusion between vertebral bodies 108. Such devices are intended to provide support to prevent excessive collapse of space between vertebrae 101 which could result in stenosis of the spinal canal or neuroforamina, progressive deformity, impairment or loss of nerve function, or pain. Such devices also provide at least one compartment to fill with bone, or material which assists in bone growth, in order to maintain close contact with vertebral bone as new bone is encouraged to bridge across the space involved.
Referring to FIG. 6, which shows a single plan view of a vertebra 101, it is known in the art that interbody devices can be inserted from several directions (indicated by arrowed lines) including posterior interlaminar approaches on both sides A, B, transforaminal or partially lateral approaches C, D, anterior approaches E, and straight lateral approaches F.
Though vertebral defect devices have proven useful in the lumbar or thoracic spine 104, 106, posterior and transforaminal placement (A, B, C, D) of any device is too dangerous in the cervical spine 102. Some cylindrical bone grafts and devices have also been associated with increased subsidence and kyphotic deformities, particularly in the cervical area. Subsidence is the sinking of devices or structural bone grafts into adjacent vertebral bodies.
Interbody devices have been constructed with polymers such as PEEK and carbon fiber/PEEK combinations. These devices have the advantage of minimal interference with future imaging studies whether by x-ray, CT scan, or MRI scan. Such devices usually have simple implanted metal markers in front and back to allow limited visualization of their position with x-rays or the like. Such devices are made with thick, vertically straight walls to provide support strength, but once they subside a small amount the straight walls offer no effective resistance to excessive subsidence. The surface area provided for fusion is also limited by the thick walls. Polymer material in current use does not allow construction of sharp edges and fixation elements and does not allow for varied shapes which might solve many of the problems with subsidence.
In the cervical spine 102, it is known in the art that the addition of an anterior plate will limit subsidence and kyphotic deformity, but this adds cost and is not always successful. Complications such as backing out of fixation screws and screw and plate breakage have been significant problems. Anterior plates are difficult to install if more than two disk levels are fused, and may add to dysphagia. In addition, such a plate occupies much of the anterior surface of the superior and inferior vertebral bodies, and there is data to suggest that proximity of the plate to the adjacent disks promotes more rapid degenerative changes. If surgery is required at an adjacent level, it is almost always necessary to remove the plate to perform the surgery, which increases complexity and morbidity when further surgery is required.
It is therefore desirable to provide a cervical fixation device designed to achieve rapid fixation while preventing excessive subsidence. The device should eliminate the need for ancillary stabilization devices such as anterior cervical plates and should be completely or nearly completely contained within the confines of the disk space. The device should have excellent support strength, but limit the amount of interference with future imaging studies.